The invention relates to a light switching device or light gate having an optical transmitter for emitting light pulses, an optical receiver for receiving the light pulses, a drive circuit for the optical transmitter having an adjusting device for the output power of the optical transmitter, a recognition circuit for the received light pulses having a generator for an indication signal proportional to the intensity of the received light pulses, and having a display for indicating a low level of received light.
Such light gates are known. In order to drive them and to trigger the switching process controlled by the light gate, use is regularly made of an integrated PLL, which on the one hand produces the clock pulse for the light pulses radiated by the light transmitter, and on the other hand compares the received light pulses with the generated light pulses with regard to frequency and phase. If a signal corresponding to the emitted pulse signal is not received, a switching signal is generated, for example a relay is switched over, in order to initiate the processes to be triggered by the light gate. A d.c. voltage signal which is proportional to the intensity of the received light pulses is present at one output of this integrated circuit (IC). In known light gates, this d.c. voltage signal is used to indicate a low intensity of the received light signals, which endangers a reliable evaluation of the received signals, and is generally ascribed to contamination of the optical transmitter and/or receiver.
The lighting up of, for example, a red light-emitting diode, effected thereby informs the user of the light gate that a new adjustment must be undertaken, generally by raising the gain of the input amplifier for the received light pulses. It is, however, also known to raise the output power of the optical transmitter in order to achieve a higher received power for the received light pulses. The readjustment is undertaken manually on the basis of the alarm signal.
The journal "Konstruktion und Elektronik" of May 10, 1989, page 14 discloses a light gate in which the drop in the signal level at the receiver is compensated for automatically by an automatic rise in the gain. Because of the reduction in the signal-to-noise ratio connected with this, the operational reliability is no longer guaranteed in the case of heavy contamination.
A light gate intended for counting banknotes is described in DE 31 24 464 A1. The banknote passes through the arrangement of an optical transmitter and an optical receiver. The device is switched on at the start of the counting process. Here, the optical transmitter is controlled in such a way that it radiates at maximum intensity. The intensity of the optical transmitter is reset in steps as a function of the signal received by the optical receiver until a prescribed received intensity is reached at the optical receiver. The transmitting power of the optical transmitter is thus adjusted in accordance with the criterion of the received light power. Since the optical transmitter is driven by a d.c. voltage, the arrangement is not suitable for normal light gates, because of its vulnerability. Useful operation results only on the basis of the short distance between the optical transmitter and optical receiver, and the interference effects connected therewith.
It is known from DE 37 33 256 A1 to vary the constant current flowing through an incandescent lamp by means of the manual adjustment of the sensitivity of an evaluation circuit for the light of the incandescent lamp received by a phototransistor. The light gate is intended for controlling a sewing machine, in which the light passes through the material. In the case of a thin material, the full light power of the incandescent lamp is not required, so that a reduction in the light power is possible by reducing the current flowing through the incandescent lamp, so that the service life of the incandescent lamp is increased.
It is known in principle, for example from DE 35 41 002 A1, to keep the output power of a light source, for example a laser light source, constant by measuring a portion of the luminous flux and comparing it with a comparison value.